The present invention relates to a semiconductor device and, more particularly, to a device having a copper pad and a method of fabricating the same.
Recently, in the field of semiconductor devices, wiring layers in a semiconductor chip are formed by using copper, instead of aluminum, for reasons of preventing signal delay and the like. In this case, pads formed on the surface of a semiconductor chip are also formed using copper like copper wiring.
The following three methods are used to mount a semiconductor chip on a wiring substrate and electrically connect them.
The first method is generally called flip chip mounting by which a semiconductor chip is mounted in a vertically inverted state on a wiring substrate. Solder bumps are formed on copper pads of the semiconductor chip. The semiconductor chip is mounted on the wiring substrate via the solder bumps, and a layer of an encapsulating resin is formed between them.
Solder balls arranged in the form of an array are formed on the opposite side of the wiring substrate and connected to a printed circuit board (not shown) or the like.
A method relevant to the present invention by which a solder bump is formed on a copper pad will be described below.
FIGS. 18A to 18E show a semiconductor device fabrication method of forming a copper pad in order of steps.
As shown in FIG. 18A, a copper pad 101 is formed on the surface of a silicon substrate 103. With the surface of this copper pad 101 exposed, the silicon substrate 103 is covered with a passivation film 102.
As shown in FIG. 18B, a titanium film 104, a nickel film 105, and a palladium film 106 are stacked in this order on the entire wafer surface by sputtering or evaporation, thereby forming a barrier metal.
As shown in FIG. 18C, this barrier metal is coated with a resist, and a hole is formed to obtain a resist film 107. In this hole, solder plating is formed as a low-melting metal film 108 for forming a projecting electrode.
As shown in FIG. 18D, the resist film 107 is removed, and the Pd/Ni/Ti films 104, 105, and 106 forming the barrier metal are etched.
The whole semiconductor wafer is coated with a flux and heated in a nitrogen atmosphere to reflow the solder.
The second method is wire bonding. As shown in FIG. 19, a copper pad 302 is formed on a silicon substrate 300. With the surface of this copper pad 302 exposed, the silicon substrate 300 is covered with a passivation film 301. A gold wire 304 is connected to the copper pad 302 of this semiconductor chip. After this bonding connection, the semiconductor chip is mounted on a wiring substrate and encapsulated with a molding resin.
The third method uses TAB (Tape Automated Bonding). That is, a gold bump is formed on a pad of a semiconductor chip. The semiconductor chip is mounted on a metal cap, and a polyimide tape on which wiring is formed is connected to the gold bump.
Unfortunately, the aforementioned semiconductor devices have the following problems. As described above, a copper pad of a semiconductor chip is subjected to (1) flip chip mounting using a solder bump, (2) connection by bonding to a gold wire, or (3) TAB mounting using a gold bump. The problems of these methods will be separately described below.
(1) To form a solder bump on a copper pad, a metal stacked film is formed to suppress diffusion of tin in the solder. However, copper in the copper pad reaches the solder through this metal stacked film and forms an intermetallic compound of tin and copper. As a consequence, the shear strength lowers when the device is left to stand at high temperatures.
(2) In bonding connection of a copper pad and a gold wire, it is difficult to connect gold and copper by ultrasonic waves commonly used.
(3) To form a gold bump on a copper pad, a metal stacked film is formed to suppress diffusion of gold. However, copper in the copper pad reaches the gold through this metal stacked film. Consequently, the shear strength lowers by diffusion of the gold and copper.
To avoid the problem of item (1) above, a thick barrier metal of copper or nickel can be formed on a copper pad by electroplating. In this method, however, the number of fabrication steps increases by the plating step.
The present invention has been made in consideration of the above situation, and has as its object to provide a semiconductor device capable of ensuring high shear strength by connecting a solder bump, gold wire, or gold bump to a copper pad without increasing the number of fabrication steps, and a method of fabricating the same.
The present invention is a semiconductor device in which a semiconductor element having a copper pad is mounted on a wiring substrate, comprising a copper diffusion preventing film formed on the surface of the copper pad to prevent diffusion of copper, and a metal bump electrically connected to the copper pad with the copper diffusion preventing film interposed between them, wherein the semiconductor element is mounted on the wiring substrate via the metal bump.
The copper diffusion preventing film can contain at least one of Ni, Cr, TiN, TaN, Ta, Nb, and WN.
The present invention is a semiconductor device in which a semiconductor element having a copper pad is mounted on a wiring substrate, comprising a copper diffusion preventing film formed on the surface of the copper pad to prevent diffusion of copper, a metal film formed on the surface of the copper diffusion preventing film to improve adhesion between the copper diffusion preventing film and a metal wire, and the metal wire electrically connected to the copper pad with the copper diffusion preventing film and the metal film interposed between them, wherein the semiconductor element is mounted on the wiring substrate via the metal wire.
The copper diffusion preventing film can contain at least one of Ni, Cr, TiN, TaN, Ta, Nb, and WN, and the metal film can contain one of Au and Pd.
The metal bump can contain gold, and one of a stacked film of Ti, Ni, and Pd, a stacked film of Ti, Ni, and Au, a stacked film of TiW and Au, and a stacked film of TiW and Pd can be formed between the copper diffusion preventing film and the metal bump.
The metal bump can contain solder, and one of a stacked film of Ti and Ni, a stacked film of Ti, Ni, and Pd, a stacked film of Ti, Ni, and Au, a stacked film of Cr and Ni, a stacked film of Cr and Au, a stacked film of Cr, Ni, and Au, a stacked film of Cr, Ni, and Pd, a stacked film of Ti and Cu, a stacked film of Ti, Cu, and Au, a stacked film of Cr and Cu, and a stacked film of Cr, Cu, and Au can be formed between the copper diffusion preventing film and the metal bump.
The present invention is a method of fabricating a semiconductor device in which a semiconductor element having a copper pad is mounted on a wiring substrate, comprising the steps of forming a copper diffusion preventing film for preventing diffusion of copper on the surface of the copper pad, forming a metal bump to be electrically connected to the copper pad with the copper diffusion preventing film interposed between them, and mounting the semiconductor element on the wiring substrate via the metal bump.
The present invention is a semiconductor device fabrication method of mounting a semiconductor element having a copper pad on a wiring substrate by flip chip mounting by using a solder bump, comprising the steps of forming a copper diffusion preventing film for preventing diffusion of copper on the surface of the copper pad, forming a metal stacked film for suppressing diffusion of tin contained in a solder bump on the copper diffusion preventing film, forming the solder bump on the metal stacked film, and mounting the semiconductor element on the wiring substrate via the solder bump.
The metal stacked film can be one of a stacked film of Ti and Ni, a stacked film of Ti, Ni, and Pd, a stacked film of Ti, Ni, and Au, a stacked film of Cr and Ni, a stacked film of Cr and Au, a stacked film of Cr, Ni, and Au, a stacked film of Cr, Ni, and Pd, a stacked film of Ti and Cu, a stacked film of Ti, Cu, and Au, a stacked film of Cr and Cu, and a stacked film of Cr, Cu, and Au.
The present invention is a method of fabricating a semiconductor device in which a semiconductor element having a copper pad is mounted on a wiring substrate, comprising the steps of forming a copper diffusion preventing film for preventing diffusion of copper on the surface of the copper pad, forming a metal film for improving adhesion to a metal wire on the copper diffusion preventing film, electrically connecting the metal wire to the copper pad with the copper diffusion preventing film and the metal film interposed between them, and mounting the semiconductor element on the wiring substrate via the metal wire.
The present invention is a method of fabricating a semiconductor device in which a semiconductor element having a copper pad is mounted on a wiring substrate, comprising the steps of forming a copper diffusion preventing film for preventing diffusion of copper on the surface of the copper pad, forming a metal stacked film for preventing diffusion of gold on the copper diffusion preventing film, forming a gold bump to be electrically connected to the copper pad with the copper diffusion preventing film and the metal stacked film interposed between them, and mounting the semiconductor element on the wiring substrate via the gold bump.
In the semiconductor devices and their fabrication methods according to the present invention, a copper diffusion preventing film formed on a copper pad suppresses diffusion of copper, and this suppresses diffusion of a component in a metal bump or metal wire and copper. Therefore, no intermetallic compound of the component in the metal bump or metal wire and copper is formed, so no interfacial de-adhesion removal takes place. Accordingly, a highly reliable connection is obtained. Additionally, the fabrication process can be simplified compared to a method of forming a thick barrier metal by electroplating.
Also, the present invention is a semiconductor device in which a semiconductor element having a copper pad is mounted on a wiring substrate, comprising a copper diffusion preventing film formed on the surface of the copper pad to prevent diffusion of copper, an aluminum film formed on the surface of the copper diffusion preventing film, and a metal wire electrically connected to the copper pad with the copper diffusion preventing film and the aluminum film interposed between them, wherein the semiconductor element is mounted on the wiring substrate via the metal wire.
The device can further comprise a metal film for improving adhesion between the copper diffusion preventing film and the aluminum film.
The copper diffusion preventing film can contain at least one of Ni, Cr, TiN, TaN, Ta, Nb, and WN.
When the device further comprises the metal film, this metal film can contain at least one of Ti, Ni, Cr, TiN, TaN, Ta, Nb, and WN.
The device can further comprise a passivation film covering the copper pad or a passivation film covering the copper pad and the copper diffusion preventing film.
A method of fabricating this semiconductor device comprises the steps of forming a copper diffusion preventing film for preventing diffusion of copper on the surface of the copper pad, forming an aluminum film on the copper diffusion preventing film, electrically connecting a metal wire to the copper pad with the copper diffusion preventing film and the aluminum film interposed between them, and mounting the semiconductor element on the wiring substrate via the metal wire.
The method can further comprise the step of forming a metal film for improving adhesion between the copper diffusion preventing film and the aluminum film.